Lightweight composite SCBA (Self-Contained Breathing Apparatus) pressurized cylinders are in wide use in the firefighting, medical, aviation and transportation markets. Composite material (S-2 glass/epoxy, Kevlar/epoxy, carbon/epoxy, etc.) are used for life support gas cylinders, aircraft inflatables, oxygen supply applications, life support cylinders, recreational SCUBA tanks 1 (FIG. 1) and alternative fuel vehicles.
Composite pressure tanks are manufactured by overwrapping aluminum liners with hoop-wrapped or full-wrapped (axial+hoop) shells. The aluminum liner serves as an impermeable gas barrier and the composite shell provides the structure that resists the pressure, typically in the range of 3,000 to 4,500 psi. The U.S. Department of Transportation is responsible for the safe use of these tanks and requires periodic hydrostatic testing to determine the structural integrity of composite tanks.
Advantages of these tanks are light weight, corrosion resistance, dimensional stability, neutral buoyancy, and the ability to store more air than an equivalent all-metal tank. There is, however, a lower confidence level with the use of composite tanks than with metal tanks. The fatigue mechanism in composite tanks that produces matrix cracking and eventual ply failures is more complex and less predictable than for metal tanks. Another problem with composite tanks is the potential for external surface damage that comes from the kind of rough handling to which SCBA/SCUBA tanks often are subjected. Safety is a key issue. The ability to monitor structural integrity under a variety of operating conditions will be an important factor that determines future tank recertification intervals and tank life.
What is needed is a means of insuring that composite tanks are as safe to operate as metal tanks.